An actuating unit of this type is generally disclosed in patent application DE 199 29 154.3. The special features of the prior-art actuating unit consist in that the travel simulator is provided by a resetting spring biasing the piston in opposition to the direction of actuation, and there is a valve device which closes or opens a hydraulic connection between the pressure chamber bounded by the piston and a non-pressurized pressure fluid supply reservoir. The mentioned publication, however, does not provide a system for detecting a driver's deceleration wish.